1. Field of the Invention
This invention relates to a recording and reproducing device for a magnetic disc, which is adapted to be used as a terminal device for a computer, word processor and the like.
2. Description of Prior Art
Heretofore, so-called steel belt type mechanisms have been widely used in the recording and reproducing device for a magnetic disc for shifting a carriage supporting a magnetic head intermittently in a diametrical direction of the magnetic disc. However, since the mechanisms of this type have difficulties in attaching and tension-adjusting the steel belt and in economizing space factor, screw-threaded shaft type mechanisms tend to be used recently instead of the steel belt type mechanisms for shifting the carriage of the recording and reproducing device because of the advantageous feature in economizing the space factor.
FIG. 1 is a plan view showing a conventional screw-threaded shaft type mechanism for shifting the carriage intermittently. The mechanism comprises a step motor 1 secured to a chassis 2, and a screw-threaded shaft 3 having a helical groove 3a on the outer surface. The shaft 3 is directly coupled to a cylindrical permanent magnet (not shown) constituting the rotor of the step motor 1 which is rotatably supported by a bearing (not shown) in the casing of the motor 1, while the other end of the shaft 3 is rotatably supported by a bearing 4. Numeral 5 designates the carriage which supports recording and reproducing magnetic head 6. The mechanism further comprises two guide shafts 7 and 8 for guiding the movement of the carriage 5, and an engaging piece 9 secured to the carriage 5. The engaging piece 9 has a conical projection 9a engaging with the helical groove 3a of the screw-threaded shaft 3.
With the above described construction, when the step motor 1 rotates the screw-threaded shaft 3 in the forward or reverse direction, the conical projection 9a engaging with the helical groove 3a of the shaft 3 is shifted in the arrow marked direction in FIG. 1, and hence the carriage 5 is reciprocated along the guide shafts 7 and 8 for a distance corresponding to the rotating angle of the screw-threaded shaft 3.
The helical groove 3a of the screw-threaded shaft 3 has a constant leading angle 0 around the circumference thereof as shown in FIG. 2, so that it can be developed into a linear line. Thus, when the movement of the carriage 5 is intermittently stopped at, for instance, eight positions corresponding to the rotating angles 0.degree., 45.degree., 90.degree., 135.degree., 180.degree., 225.degree., 270.degree. and 315.degree. of the screw-threaded shaft 3, the position of the carriage 5 tends to be deviated depending on the irregularity in the inertia of the carriage or in the control of the step motor 1, thus adversely affecting to the accuracy of recording and reproduction of the magnetic head 6.